This invention pertains to room thermostats which are intended to control temperature within a confined space. More particularly, this invention pertains to an improved thermostat control system for a heating and/or cooling system.
Thermostats are designed to control temperature within a confined space. They typically consist of a temperature sensing device, usually a bi-metal member, an automatic switch, such as a mercury switch or switches mounted on the bi-metal member to control a heating and/or cooling system, and manual switches to select the operational mode (heat, cool, or off) and blower operation. Because of the necessary mass of the bi-metal member, it is slow to respond to rapid changes of the air temperature surrounding it. If left uncorrected, this will result in large oscillations of room temperature about the desired temperature setting.
This problem has been addressed in a relatively effective manner in the prior art. A resistor is thermally linked to the bi-metal member and when heat is added to the confined space, heat is simultaneously additionally added to the bi-metal member by electrically energizing the resistor. In this manner the bi-metal member of the thermostat "anticipates" the heating of the confined space and terminates the heating sooner than it otherwise would. The resistor thermally linked to the bi-metal member is commonly called an "anticipator". As the outdoor temperature decreases, the need for heat in the confined space increases and the bi-metal temperature increases above room temperature. In other words, the control temperature decreases because the bi-metal member is warmer than the space temperature. This results in a drop in room temperature or an effect called "droop". In a more sophisticated sense, droop is defined as the change in control temperature between 20 percent load (on time) and 80 percent load (on time). Droop is undesirable as it reduces the comfort of the inhabitants of the area to be conditioned. The discomfort effect is magnified by the normal reduction in humidity as the outdoor temperature decreases and is reflected in the confined space. My prior U.S. Pat. No. 3,948,438 granted Apr. 6, 1976 discloses a thermostat system which includes an anticipator capable of dissipating heat at a substantially constant rate over a range of applied voltages and current. The anticipator is made from a positive temperature coefficient material. However, U.S. Pat. No. 3,948,438 does not suggest the inventive thermostat with enhanced outdoor temperature anticipation of this invention.
A two-stage thermostat will typically control to over twice the change in room temperature as compared to a single stage thermostat. This is due to the required interstage differential and the change due to the droop in each stage.
The prior art has offered a partial solution to the problem of the change in control point which may be on the order of 8.degree. F. One solution consists of an ambient compensating negative temperature coefficient (NTC) thermistor physically located in the outdoor ambient air (O.D.A.) and responsive to changes in that air temperature, and connected electrically in series with an outdoor ambient air fixed resistor thermally linked to the thermostat bi-metal member The NTC thermistor has a non-linear negative exponential resistance versus temperature characteristics to provide a large change in resistance for a narrow temperature change. The two devices in series, i.e., the NTC thermistor and the fixed resistor, are continuously powered by the 24 volt transformer. Theoretically, the thermostat bi-metal member then receives a lesser amount of heat from the fixed resistor as the anticipators add more heat with a decreasing outdoor ambient. The thermostat can be "off-calibrated" such that the thermostat will be in apparent calibration over the range of "percent heating capacity" required for comfort.
A large number of heat pumps are installed in climates with a minimum outdoor temperature (design temperature) on the order of 30.degree. F., for example, in Florida or in Arizona. In such environments, there is not enough outdoor temperature change to effect an appreciable change in the resistance of the negative temperature coefficient (NTC) thermistor. In turn, it then cannot change appreciably the wattage in the fixed resistor, resulting in improper control.
Good design of heating and/or cooling equipment attempts to match the capacity of the heating and/or cooling equipment, e.g., heat pump, or perhaps furnace and air conditioner, with the calculated load as closely as possible. It is desired to minimize short cycling. An outdoor heat pump or air conditioner matched with a given indoor coil will remove a certain amount of moisture at a given air volume and return air dry/wet bulb temperature. It is important that the indoor air handler used provides the proper air flow volume. Further, the indoor air handler or blower fan should not move the air with excessive velocity, for then the inhabitant of the area being conditioned might be uncomfortable due to the draft.
Another important consideration is the thermostat setting and control for achieving comfort. For example, in an application where the humidity is high, a lower thermostat setting may be needed to remove enough moisture to obtain comfort conditions. This will allow the air conditioner to run longer and remove more moisture from the air. Raising the thermostat setting to reduce energy bills will result in high humidity if latent loads are high.
The present invention is intended to work well within the proper design considerations for a heating and/or cooling system or heat pump and to provide a thermostat with enhanced outdoor anticipation to provide comfort for the user, whether in a Northern clime in the United States, for example, where the minimum outdoor ambient is below zero degrees F. down to -40.degree. F., or in a Southern clime in the United States, where the minimum outdoor ambient is on the order of 30.degree. F.
An object of the present invention is to provide an improved thermostat which overcomes the noted disadvantages and deficiencies of prior thermostats.
Another object of the present invention is to provide an improved thermostat for a heat pump or for one, or two stage heating, one or two stage cooling applications having a positive temperature coefficient thermistor and a variable resistor in series with one another paralleling a fixed resistor within the thermostat, and these devices in series with a negative temperature coefficient thermistor, with the positive and negative temperature coefficient thermistors sensing the outdoor ambient temperature for providing a more reliable thermostat for a wide range of ambient conditions.
A further object of the present invention is to provide an improved thermostat for heat pumps or single or multi-stage heating and/or cooling systems which incorporates enhanced outdoor temperature anticipation to provide significantly enhanced comfort to the user. Other objects and advantages of the present invention will be made more apparent hereinafter.